Upgraded to AppleCare One for my iPhone and Watch, with another Apple Account on which the previous Apple Care+ was purchased with. After upgraded to AppleCare One, signed in to my regular Apple Account, soon after received this message:
A new Apple Account has signed in to your iPhone 14 Pro Max. To avoid losing AppleCare One coverage, you must sign in to the iPhone 14 Pro Max with the Apple Account associated with your AppleCare One plan.
You have 24 hours from when this email was sent to sign in, or your iPhone 14 Pro Max will be removed from your AppleCare One plan.
This is an important change which they do not communicate clearly about. So be careful if you want to change the Apple Account on your device later.